Return Value

Override MeasureOverride to implement custom layout sizing behavior for your element as it participates in the Windows Presentation Foundation (WPF) layout system. Your implementation should do the following:

Iterate your element's particular collection of children that are part of layout, call Measure on each child element.

Immediately get DesiredSize on the child (this is set as a property after Measure is called).

Compute the net desired size of the parent based upon the measurement of the child elements.

The return value of MeasureOverride should be the element's own desired size, which then becomes the measure input for the parent element of the current element. This same process continues through the layout system until the root element of the page is reached.

During this process, child elements might return a larger DesiredSize size than the initial availableSize to indicate that the child element wants more space. This might be handled in your own implementation by introducing a scrollable region, by resizing the parent control, by establishing some manner of stacked order, or any number of solutions for measuring or arranging content.

Important Note:

Elements should call Measure on each child during this process, otherwise the child elements will not be correctly sized or arranged.

Notes to Inheritors:

The following non-compiling code shows this implementation pattern. VisualChildren represents an enumerable collection property of children that your own element should define. The property can be named anything. VisualChildren is a placeholder name for purposes of this example, VisualChildren is not an API as provided by WPF or a part of a naming pattern..

protectedoverride Size MeasureOverride(Size availableSize)
{
Size desiredSize = new Size();
foreach (UIElement child in VisualChildren)
{
child.Measure(availableSize);
// do something with child.DesiredSize, either sum them directly or apply whatever logic your element has for reinterpreting the child sizes // if greater than availableSize, must decide what to do and which size to return
}
// desiredSize = ... computed sum of children's DesiredSize ...; // IMPORTANT: do not allow PositiveInfinity to be returned, that will raise an exception in the caller! // PositiveInfinity might be an availableSize input; this means that the parent does not care about sizing return desiredSize;
}

This example shows how to override the default layout behavior of the Panel element and create custom layout elements that are derived from Panel.

The example defines a simple custom Panel element called PlotPanel, which positions child elements according to two hard-coded x- and y-coordinates. In this example, x and y are both set to 50; therefore, all child elements are positioned at that location on the x and y axes.

To implement custom Panel behaviors, the example uses the MeasureOverride and ArrangeOverride methods. Each method returns the Size data that is necessary to position and render child elements.